When Senator Bernie Sanders thanked supporters for his landslide victory in the New Hampshire Democratic primary, he wistfully reminisced about his upbringing as “the son of a Polish immigrant who came to this country speaking no English and having no money.”

While the crowd cheered, Rabbi Michael Paley of New York was among many Jews watching the speech who were taken aback. He said he was surprised that the Vermont senator had not explicitly described his father as a “Polish Jewish immigrant,” a significant distinction given Poland’s checkered history with its Jewish population.

“Nobody in Poland would have considered Bernie a Pole,” Rabbi Paley said.
Two days later, in a debate with Hillary Clinton, Mr. Sanders referred to the historic candidacy of “somebody with my background” without overtly saying he was Jewish. That prompted the Jewish Telegraphic Agency, a news service feeding the Jewish press worldwide, to ponder, as its headline put it, “People are confused why Bernie Sanders won’t own his Jewishness.”

Mr. Sanders, those who know him say, exemplifies a distinct strain of Jewish identity, a secular offshoot at least 150 years old whose adherents in the shtetls of Eastern Europe and the jostling streets of the Lower East Side were socialists, anarchists, radicals and union organizers focused less on observance than on economic justice and repairing a broken world. Indeed, he seems more comfortable speaking about Pope Francis, whose views on income inequality he admires, than about his own religious beliefs.

Rabbi Paley, who worked with Jews in central Vermont when he was a Dartmouth College chaplain, recalled once talking with Mr. Sanders about “non-Jewish Jews,” a term coined by a leftist biographer, Isaac Deutscher, to describe those who express Jewish values through their “solidarity with the persecuted.” Mr. Sanders seemed to acknowledge that the term described him, Rabbi Paley said.

But the secular image that Mr. Sanders casts is also complicating the way American Jews regard the historic nature of his candidacy.

When Joseph I. Lieberman, an Orthodox Jew who spurned campaigning on the Sabbath, was Al Gore’s vice-presidential running mate in 2000, many Jewish voters saw it as a breakthrough. While Mr. Sanders’s surprising run for even higher office is eliciting many strong emotions, religious pride is usually not the main one.

“Joe was an observant Jew; Bernie is marginal,” said Morris Harary, a lawyer who lives near Mr. Sanders’s childhood home in Brooklyn. As a history maker, he said, Mr. Lieberman was “much more of a big deal.”

Growing up in the Midwood neighborhood in the 1940s and ’50s, Mr. Sanders, who declined to be interviewed for this article, had a not atypical Jewish upbringing. His father, Eli, who sold paint to hardware stores, showed up at a synagogue virtually only on Yom Kippur, Mr. Sanders’s brother, Larry Sanders, said in an interview from England, where he is the health issues spokesman for the Green Party.

Their mother, Dorothy, was the daughter of a union activist who chafed at his own yeshiva schooling. The family did not observe much more than Passover seders with neighbors.

“They were very pleased to be Jews, but didn’t have a strong belief in God,” Larry Sanders said.

Like many children of that era, Bernie Sanders, while attending public schools, took Sunday Hebrew and Bible classes at an Orthodox synagogue, the Kingsway Jewish Center in the Midwood neighborhood, and was bar mitzvahed there.

After graduating from the University of Chicago, Mr. Sanders went to Israel to work on an agricultural kibbutz and ended up at Sha’ar Ha’amakim (Gate of the Valleys) near Haifa. The motivation seemed as much ideological — the collective was affiliated with the Hashomer Hatzair socialist movement — as Zionistic.

As the mayor of Burlington, Vt., Mr. Sanders in 1983 was asked by Rabbi Yitzchok Raskin to permit the lighting of an eight-foot-tall menorah on the steps of City Hall. He not only agreed but lit the second-night candles himself. Rabbi Raskin recalled that when he asked Mr. Sanders if he needed guidance, Mr. Sanders said, “I know the blessings,” and recited them in Hebrew.

[Photo As a child, Mr. Sanders took Sunday Hebrew and Bible classes at Kingsway Jewish Center, an Orthodox synagogue in Brooklyn. Credit Hilary Swift for The New York Times]

As important to Mr. Sanders’s outlook was the Holocaust’s impact on his family. Three of his father’s siblings — two brothers and a sister — were slaughtered by the Germans, and other relatives perished.

In 2013, the Sanders brothers traveled to their father’s hometown, Slopnice, a rural village 50 miles southeast of Krakow, a visit tinged with nostalgia as well as sadness.

The thousand-year history of Jews in Poland was marked by periods of tolerance and well-being but also by waves of brutal anti-Semitism. Many Poles hid and rescued Jews during the Holocaust, but others aided in the Nazi campaign that killed 90 percent of Poland’s Jews.

Mr. Sanders was forever mindful, as he once said, that the appointment of Hitler as Germany’s chancellor in 1933 “ended up killing 50 million people around the world,” six million of them Jews.

“Bernie learned that politics is a very serious matter,” Larry Sanders said.

Today, Senator Sanders does not regularly attend any synagogue in Washington or Vermont, though he does show up for rituals like the yahrzeit — the anniversary of a death — of the father of a close friend, Richard Sugarman, who teaches philosophy in the religion department at the University of Vermont.

As a senator, Mr. Sanders has supported a two-state solution guaranteeing Israel’s right to exist as well as a Palestinian homeland, and colleagues in Congress say his view tends to echo the Israeli left wing. When Hamas fired rockets from Gaza into Israeli towns, he condemned the attacks, but he also criticized Israel for what he said was a disproportionate military response.

He supported last year’s deal to end sanctions against Iran in exchange for its dismantling of the infrastructure the United States believed would give it the capacity to make nuclear bombs. Some Jewish members of Congress, notably Senator Chuck Schumer of New York, criticized the deal as not doing enough to stop Iran’s nuclear development and thus putting Israel at risk.

As a presidential candidate, Mr. Sanders nettled some Jews by making a campaign appearance on Rosh Hashana, a day most Jews take off from work, at Liberty University, an evangelical college in Virginia founded by the Rev. Jerry Falwell. But he appeared later that day with Lynchburg’s mayor for the Rosh Hashana ritual of tashlikh, the symbolic casting of sins into a stream.

Unlike Mrs. Clinton, who states emphatically that she is a Methodist “person of faith,” Mr. Sanders responds to questions about his beliefs by turning the conversation toward political ideals.

In October, Mr. Sanders was asked on “Jimmy Kimmel Live!” whether he believed in God.

[Photo In a rare and playful nod to his Jewish background, Mr. Sanders appeared as “Bernie Sanderswitzky” in a sketch on “Saturday Night Live” with the guest host, Larry David. Credit Dana Edelson/NBC]

“What my spirituality is about is that we’re all in this together and it’s not a good thing to believe that as human beings we can turn our backs on the suffering of other people,” he responded. “This is not Judaism. This is what Pope Francis is talking about, that we cannot worship just billionaires and the making of more money.”

Yet he playfully acknowledged his Jewish background in a recent “Saturday Night Live” sketch where he took the part of an ocean-crossing immigrant named Bernie Sanderswitzky. “We’re going to change it when we get to America so it doesn’t sound quite so Jewish,” he told the host, Larry David, in his conspicuous Brooklyn accent.

“Yeah, that’ll trick ’em,” Mr. David replied.

Rabbi Joshua Chasan, the rabbi emeritus of Burlington’s Conservative synagogue, Ohavi Zedek, who has known Mr. Sanders since he was Burlington’s mayor, said Mr. Sanders “does not have to wear his Judaism on his sleeve in Vermont or anywhere else to be a Jew.”

But Rabbi Chasan said there was probably another reason Mr. Sanders avoided discussion of his religion. Like John F. Kennedy, who had to overcome anti-Catholic sentiment, “he’s a good politician and he knows what he’s up against,” he said.

Still, Mr. Sanders’s Jewishness, even in its secular style, may not be much of an issue. A Gallup poll last year found that 92 percent of Americans said they would vote for a Jewish president, double the percentage from when Gallup first asked the question in 1937, and roughly the same as those who said they would vote for a woman, an African-American, a Catholic or a Hispanic. (Only 47 percent, however, said they would vote for a socialist.)

Jewish voters, consequently, may now feel less obligation to support a fellow Jew, another possible reason for the muted excitement about Mr. Sanders. Kevin Greenberg, 52, a native of Wilkes-Barre, Pa., said much had changed since he was a young man in 1986 and was told by a family friend to vote for Marc Holtzman, a Republican running for Congress in Pennsylvania, “because he was Jewish.”

Still, there are lingering fears of rejection, or worse.

“It’s very uplifting to me,” Estelle Berman, an 89-year-old Bronx native living in Delray Beach, Fla., said of Mr. Sanders’s run. “But Middle America will never accept him.”

And some Jewish voters believe it would not take much for anti-Semitism to surface in the United States, as it has in corners of Europe.

Mr. Greenberg’s 22-year-old son, Josh, who plans to become a rabbi and is a senior at Gettysburg College in Pennsylvania, said of Mr. Sanders’s Jewish background, “Some hateful people might bring that up.”

“It wouldn’t be hard,” he added, “for it to be considered a bad thing.”